Merge pull request reduxjs#1180 from Shrugsy/docs/add-rtkq-to-readme

📝 Add RTKQ section to README

Author: markerikson

README.md

@@ -50,7 +50,9 @@ The **Redux Toolkit** package is intended to be the standard way to write Redux
 
 We can't solve every use case, but in the spirit of [`create-react-app`](https://github.com/facebook/create-react-app) and [`apollo-boost`](https://www.apollographql.com/blog/announcement/frontend/zero-config-graphql-state-management/), we can try to provide some tools that abstract over the setup process and handle the most common use cases, as well as include some useful utilities that will let the user simplify their application code.
 
-Because of that, this package is deliberately limited in scope. It does _not_ address concepts like "reusable encapsulated Redux modules", data caching, folder or file structures, managing entity relationships in the store, and so on.
+Because of that, this package is deliberately limited in scope. It does _not_ address concepts like "reusable encapsulated Redux modules", folder or file structures, managing entity relationships in the store, and so on.
+
+Redux Toolkit also includes a powerful data fetching and caching capability that we've dubbed "RTK Query". It's included in the package as a separate set of entry points. It's optional, but can eliminate the need to hand-write data fetching logic yourself.
 
 ## What's Included
 
@@ -64,6 +66,33 @@ Redux Toolkit includes these APIs:
 - `createEntityAdapter`: generates a set of reusable reducers and selectors to manage normalized data in the store
 - The `createSelector` utility from the [Reselect](https://github.com/reduxjs/reselect) library, re-exported for ease of use.
 
+## RTK Query
+
+**RTK Query** is provided as an optional addon within the `@reduxjs/toolkit` package. It is purpose-built to solve the use case of data fetching and caching, supplying a compact, but powerful toolset to define an API interface layer for your app. It is intended to simplify common cases for loading data in a web application, eliminating the need to hand-write data fetching & caching logic yourself.
+
+RTK Query is built on top of the Redux Toolkit core for its implementation, using [Redux](https://redux.js.org/) internally for its architecture. Although knowledge of Redux and RTK are not required to use RTK Query, you should explore all of the additional global store management capabilities they provide, as well as installing the [Redux DevTools browser extension](https://github.com/reduxjs/redux-devtools), which works flawlessly with RTK Query to traverse and replay a timeline of your request & cache behavior.
+
+RTK Query is included within the installation of the core Redux Toolkit package. It is available via either of the two entry points below:
+
+```ts no-transpile
+import { createApi } from '@reduxjs/toolkit/query'
+
+/* React-specific entry point that automatically generates
+   hooks corresponding to the defined endpoints */
+import { createApi } from '@reduxjs/toolkit/query/react'
+```
+
+### What's included
+
+RTK Query includes these APIs:
+
+- `createApi()`: The core of RTK Query's functionality. It allows you to define a set of endpoints describe how to retrieve data from a series of endpoints, including configuration of how to fetch and transform that data. In most cases, you should use this once per app, with "one API slice per base URL" as a rule of thumb.
+- `fetchBaseQuery()`: A small wrapper around fetch that aims to simplify requests. Intended as the recommended baseQuery to be used in createApi for the majority of users.
+- `<ApiProvider />`: Can be used as a Provider if you do not already have a Redux store.
+- `setupListeners()`: A utility used to enable refetchOnMount and refetchOnReconnect behaviors.
+
+See the [**RTK Query Overview**](https://redux-toolkit.js.org/rtk-query/overview) page for more details on what RTK Query is, what problems it solves, and how to use it.
+
 ## Documentation
 
 The Redux Toolkit docs are available at **https://redux-toolkit.js.org**.